A typical ink jet printer, plotter, or other printing system has a pen that reciprocates over a printable surface such as a sheet of paper. The pen includes a print head having an array of numerous orifices through which droplets of ink may be expelled into the surface to generate a desired pattern. Color ink jet printers typically employ four print heads, each connected to an ink supply containing a different color of ink (e.g. black, cyan, yellow, and magenta.) The different print heads may be included on separate, replaceable ink pens. A full color image may be printed by sequentially printing overlapping patterns with each of the different color inks. For good printed output, the different color images must be in precise registration.
In existing printers, registration of the different colors may be achieved by printing an alignment pattern with each color, then optically sensing the positions of the printed patterns and determining the amounts of any deviations from nominal positions. The printer electronically adjusts the firing position for each color so that the resulting output is registered. This is particularly critical for plotters printing on large media sheets, in which small errors may accumulate to provide unacceptable output.
To sense the position of the alignment patterns, an existing printer uses an optical module mounted to the reciprocating print head. The module has a light emitting diode (LED) illuminating a selected region of the media sheet. The light from the illuminated region is focused by a lens onto a photodetector. As the module scans across the sheet over a printed bar pattern, the photodetector records a momentary reduction in collected light flux. The printer electronics calculate the location of the printed pattern, by comparing with an electronic signal from a motion encoder that records the position of the carriage relative to the printer.
A first disadvantage of existing photosensor modules is size. The arrangement of illuminator and detector creates a bulky package, as the detector and lens must be on an axial optical path normal to the selected region, and the light source is thus offset at an angle from the optical path, providing illumination obliquely. As the illuminator is at some distance from the selected region, its remote extremities are undesirably widely spaced apart from the photodetector, creating a bulky package, which is particularly problematic for a carriage mounted component; clearance must be provided along the entire carriage path. If the module is added to the ink jet pen to increase the width of the carriage along the carriage scan axis, the entire printer width must be increased by two times the width increase to permit sensing and printing to the extreme edges of the paper. Such printer size increases are contrary to the normal goal of minimizing desktop printer housing sizes.
A second disadvantage of existing photosensor modules concerns the tradeoff between uniformity of illumination and intensity of illumination. Uniform illumination of the selected region is needed to prevent variations as being interpreted as positional errors. To improve uniformity the LED may be positioned at a greater distance, and its light transmitted through the bore of a white tube. However, the scattering of unfocused light may illuminate a larger area than required, wasting light flux. To obtain useful contrast levels for accurate measurements, a higher intensity of illumination is required to compensate for the lost light, increasing component costs and power consumption. Sharply focusing the LED's light onto the selected region achieves efficiency, but has unacceptable uniformity.
The present invention overcomes or reduces the disadvantages of the prior art by providing an optical sensor module for identifying characteristics of printed ink jet images on printing media residing in a media plane. The module has a chassis and a connected illumination source and detector spaced apart from the media plane. An integral single cluster optical element is positioned between the image plane and the illumination source and detector. The optical element has a first portion having a first optical characteristic positioned on a first optical path between the illumination source and a selected region of the media plane, and a second portion having a second optical characteristic different from the first optical characteristic and positioned on a second optical path between the illumination source and the selected region. The optical element may include diffractive optics, fresnel lenses, and conventional lenses formed of transparent plastics to steer, focus and diffuse light onto the selected region and return it efficiently to the detector.